Does QFT specify particle propagation?

Click For Summary

Discussion Overview

The discussion revolves around the nature of particle propagation in Quantum Field Theory (QFT) and its relationship to concepts of kinetic energy and spacetime. Participants explore whether QFT specifies how particles propagate through spacetime or merely describes their existence and interactions without a clear spatial trajectory.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • Some participants question whether QFT shows how a particle propagates through spacetime or if it only describes particle propagation through time without spatial reference.
  • One participant recalls that QFT primarily concerns itself with particle creation and interactions, suggesting that it does not specify exact spatial trajectories.
  • Another participant asserts that QFT can describe how a particle moves from one point to another in spacetime, indicating that integrals over spacetime are often performed in calculations.
  • A later reply seeks clarification on the meaning of "propagate," suggesting that QFT does account for time evolution of wave-like fluctuations in quantum fields.
  • Concerns are raised about the relationship between QFT and kinetic energy, with some participants questioning whether QFT addresses kinetic energy in the context of particle propagation.
  • Some participants argue that discussing velocity in QFT is problematic, as particles are treated as waves with defined momentum or position, complicating the notion of moving from point A to point B.
  • There is a discussion on the implications of kinetic energy in QFT, with one participant suggesting that while a single quantum fluctuation has definite energy, it is not localized, similar to quantum mechanics.
  • Feynman diagrams are mentioned as a visualization tool for calculations in perturbation theory, with a distinction made that they do not represent the actual calculations or the entirety of QFT.

Areas of Agreement / Disagreement

Participants express differing views on whether QFT specifies particle propagation through spacetime or merely describes interactions without clear spatial trajectories. There is no consensus on the relationship between QFT and kinetic energy, and the discussion remains unresolved regarding the implications of velocity in QFT.

Contextual Notes

Limitations include the ambiguity in defining "propagation" and the challenges of localizing particles in QFT. The discussion also highlights the complexity of relating kinetic energy to QFT descriptions, with unresolved mathematical steps regarding the evolution of states and their localization.

  • #31
mfb said:
It does not. You cannot count vacuum fluctuations.
Just a moment. Isn't that what the cosmological constant is all about - the amount of quantum fluctuations in a certain volume of space?

If so, then consider that an empty box in a boosted frame is shrunk in the direction of motion. So what are we to think of the space inside the box. Does the box shrink but the space inside it does not? Then if the space inside the boosted box feels the same cosmological constant in a smaller space, then what can that mean except that a still observer would see the same amount of vacuum fluctuations inside a smaller volume?
 
Physics news on Phys.org
  • #32
friend said:
Isn't that what the cosmological constant is all about - the amount of quantum fluctuations in a certain volume of space?
No.
friend said:
If so, then consider that an empty box in a boosted frame is shrunk in the direction of motion. So what are we to think of the space inside the box. Does the box shrink but the space inside it does not? Then if the space inside the boosted box feels the same cosmological constant in a smaller space, then what can that mean except that a still observer would see the same amount of vacuum fluctuations inside a smaller volume?
Those questions do not make sense.
 
  • #33
mfb said:
No. Those questions do not make sense.
Anything and everything inside the empty boosted box would appear squeezed in the direction of motion, including the subatomic particles, and I assume any vacuum fluctuations that affect those squeezed subatomic particles. I recall an animation of particles colliding in the LHC. It showed the (what, point particles?) flattened out like two pancakes as they approached each other. If quantum fluctuations had any effect at all on these moving particles, then shouldn't we assume they'd be flattened out as well?
 
  • #34
A vacuum in a box does not have a velocity. It is a vacuum.
friend said:
If quantum fluctuations had any effect at all on these moving particles, then shouldn't we assume they'd be flattened out as well?
No, that does not make sense. How often do I have to repeat that?
 
  • #35
friend said:
Perhaps vacuum fluctuations look the same at any constant speed. I know acceleration does funny things to the vacuum fluctuations. But if every inertial observer sees every other inertial observer's vacuum fluctuations to be the same as his, then I have my answer. Just as a fast moving yard stick appears shorter to those not moving, must the vacuum fluctuations of a fast mover be different than for a still mover? If not, then no, QFT cannot explain kinetic energy; that's just put in by hand as the energy of the input states.

I would honestly suggest that you stick to the conceptual objects of "quantum particle" and "quantum field," before you traipse off into discussions using (very antiquated) terminology that you almost certainly don't understand. Vacuum fluctuations (like "holes" in the "Dirac sea") are a way of giving a picture to certain subcalculations of an observable, but the picture is pretty meaningless and just words to dress around a calculation.

If you want to take QFT seriously, you need to start by treating the quantum field as fundamental, learn the language you need to describe it, see how the Fock space structure of free QFTs leads to an infinite tower of states of N identical particles, and discover what the fundamental, physical observables of the theory are.

A fast moving yard stick appears shorter to those not moving. So must the vacuum fluctuations seen by a fast moving observe be different than the vacuum fluctuations seen by a motionless observer? Would we say that the vacuum fluctuations seen by a fast moving observer are squeezed WRT us? I take it that there is so many quantum fluctuation per meter for any observer, and the fast moving meter is squeezed. So wouldn't we say that the fast moving observer must be experiencing a squeezed version of the vacuum fluctuations. Does this make any sense?

Let's take a step back. The vacuum is the state of zero particles. (If you accelerate, you'll see a bath of particles, but it's a coordinate artifact. Let's assume we're in a Lorentz frame.) When I say "fluctuations in the quantum field" what I mean, to be clear, is exciting a particle state. If I turn on a source, that will excite the vacuum, and generate particles. Meaning you will no longer be in the vacuum state.

friend said:
I think I'm getting closer to the right language. A photon on average will encounter so many vacuum fluctuations per meter. The meter is defined by the propagation of light in a vacuum. A meter stick traveling close to the speed of light will appear shorter because ALL processes are squeezed in the direction of motion. Does this mean we still observers see their vacuum fluctuations squeezed as well in the direction of their motion? Since all processes are defined by propagation through the vacuum, And their meter sticks appear shorter, does that mean that their vacuum appears shorter as well?

No, I don't see the statement here. You seem like you need to spend more time reading about QFT before you can ask full, coherent questions on QFT. I recommend reading Tong's intro to QFT, it's free and you can find it with a google search. If you make it through the first two chapters, I think you'll be able to answer many of your own conceptual confusions. Zee's QFT in a Nut Shell might also be a useful read.
 
  • #36
Thank you for your advice. But what I'm hearing from this is basically, "Shut up and calculate". Although, you are being nice about it. I have no commitment at this time to a particular interpretation. I'm just here to gain understanding. I think that requires some visualization of what's going on in the math. And the only visualization technique for quantum theory seems to be that of virtual particles, unless, of course, you know of a better way to visualize what's going on.
 
  • #37
Let me be more direct - even if that means being less nice. You are dismissing a correct view in favor of an incorrect view because you like it better. This not science.
 
  • Like
Likes   Reactions: vanhees71
  • #38
Vanadium 50 said:
Let me be more direct - even if that means being less nice. You are dismissing a correct view in favor of an incorrect view because you like it better. This not science.
You are attributing a commitment on my part based only on my asking questions. I think this thread is done.
 
  • #39
friend said:
I think this thread is done.
Me too.
 

Similar threads

Graduate Does the Z boson pole show up in the photon propagator?
  • · Replies 5 ·
Replies
5
Views
2K
Graduate Does Causality in QFT Require Commuting Field Operators Outside the Light Cone?
  • · Replies 18 ·
Replies
18
Views
3K
Graduate What does Schwarz's QFT theorem say about poles in Green's functions?
  • · Replies 8 ·
Replies
8
Views
2K
Graduate Question about Propagators in QFT
  • · Replies 13 ·
Replies
13
Views
3K
Graduate How can you tell the spin of a particle by looking at the Lagrangian?
  • · Replies 49 ·
2
Replies
49
Views
7K
Graduate Physical properties of the vacuum in GR vs. QFT
  • · Replies 15 ·
Replies
15
Views
1K
High School What does QFT really predict about matter/anti-matter interactions?
  • · Replies 6 ·
Replies
6
Views
2K
Graduate What does "solving a quantum mechanics problem" mean?
  • · Replies 3 ·
Replies
3
Views
1K
Graduate How does one derive the Lagrangian densities used in QFT?
  • · Replies 6 ·
Replies
6
Views
3K
Undergrad The claymath 4-d QFT problem and virtual particles (as an example)
  • · Replies 36 ·
2
Replies
36
Views
4K